It is the purpose of this work to clarify the nature of molecules responsible for converting a normal cell to its malignant form by studying the nature of heat-sensitive (hs) or cold-sensitive (cs) mutants of Chinese hamster lung cells affected in the expression of the transformed state. As one of the approaches to the above purpose, it was examined whether there is complementation between hs-mutants or cs-mutants, which have previously been isolated by us. The resistance to ouabain or 6-thioguanine was introduced into these mutants as a selective marker and many cell hybrids were obtained from two different mutants using techniques of cell fusion and selective growth. Examination of temperature-dependency to express the transformed phenotype and karyotype of the cell hybrids suggest that there is a complementation in the expression of the transformed state at non-permissive temperature between hs-mutants.